1. Field of the Invention
The present invention relates to the field of optics. More particularly, the present invention relates to a circuit for detecting optical pulses that contain a small number of photons.
2. Description of the Related Art
Detection of optical pulses at the level of a single photon is important for many scientific and engineering applications, such as optical communications, quantum cryptography, time-resolved spectroscopy and quantum optics.
A semiconductor device known as an avalanche photodiode (APD) can be used for single-photon counting and/or for triggering a sequence of events that are time-dependent upon the incidence of a photon. See, for example, S. Cova et al., Review of photon counting with APD's, Appl. Opt., 35(12), p. 1956, 1996. A silicon APD provides detection of visible and near-infrared light to about 1 .mu.m wavelength. Both Ge- and InGaAs-type APD's are used for detecting light at 1.3 .mu.m, one of the two preferred telecommunications wavelengths, while only InGaAs-type APD's are suitable for detecting light at 1.55 .mu.m, the other preferred telecommunications wavelength. Photomultipliers (PMTs) and multichannel plate (MCP) detectors are also used for detecting single photons and/or for triggering a sequence of events that are time-dependent upon the incidence of a photon. Time gating the gain of any of these devices can provide discrimination with respect to scattered light and information about the time variation of the light intensity.
When an APD is biased above its reverse breakdown voltage V.sub.br in the so-called "Geiger mode", a single photon that is absorbed by the APD excites a single conduction electron, which gains sufficient energy due to the bias voltage to excite secondary conduction electrons by collision cascade. The secondary conduction electrons can, in turn, excite more electrons and so forth, resulting in a current avalanche that can provide an electronic gain on the order of 10.sup.5 -10.sup.8.
Such an enormous gain can be used for generating charge pulses on the order of picoCoulombs (pC) within a few nanoseconds, producing corresponding voltages across a 50 Ohm load resistor on the order of millivolts to tens of millivolts that can readily be sensed by conventional electronic circuitry. Generally, the voltage pulses are amplified and applied to a discriminator that outputs a pulse only when an input voltage pulse exceeds a predetermined threshold level. Discrimination helps eliminate spurious counts, and provides an output pulse having fixed characteristics suitable for triggering counters or other electronic apparatus.
After an electron avalanche has been triggered by an incident photon, the current must be turned off, or quenched, and the APD restored to a state in which it is again sensitive to an incident photon. Several quenching schemes have been proposed. For example, active quenching techniques are disclosed by R. G. W. Brown et al., Active quenching of Si APD's, Appl. Opt., 26(12), p. 2383, 1987. Passive quenching techniques are disclosed by, for example, R. G. W. Brown et al., Passive quenching of Si APD's, Appl. Opt., 25(22), p. 4122, 1986, and by P. C. M. Owens et al., Passive quenching of Ge APD's, Appl. Opt., 33(30), p. 6895, 1994.
In active quenching, additional electronic circuitry monitors the onset of an avalanche, and terminates the avalanche as quickly as possible by dropping the bias voltage below the reverse breakdown voltage V.sub.br of the APD. In passive quenching, the current resulting from an avalanche flows through a quenching resistor, causing a voltage drop that momentarily reduces the bias voltage below the reverse breakdown voltage V.sub.br of the APD. Once the avalanche current stops flowing, both types of quenching schemes restore the bias to a level above V.sub.br as quickly as possible so that the APD is in a state in which it is sensitive to an incident photon.
When photon arrival time is arbitrary, restoring the bias level of the APD as quickly as possible minimizes the possibility that an incident photon will be missed. On the other hand, maintaining a constant bias above the reverse breakdown voltage makes an APD susceptible to avalanches triggered by thermally generated carriers, giving rise to a "dark count" rate. This rate can be reduced by cooling the device. Maintaining constant bias also makes the APD sensitive to spurious photons that may be incident upon the APD. Applying a continuous bias also limits how far above breakdown the device can be biased. A higher bias voltage leads to a higher quantum efficiency and faster response time, but with the drawback of a higher dark count rate.
In some applications, the potential arrival times of photons are accurately predictable, or a narrow time window of sensitivity is desired. In such cases, it is advantageous to use a "pulsed biasing" technique for detecting ultra-weak optical pulses. For pulsed biasing, the bias voltage of an APD detector is only raised above V.sub.br during the intervals of time when photon arrival is anticipated. Between these intervals the detector bias is reduced, suppressing both generation of dark counts and triggering caused by spurious photons. Pulsed biasing has significant advantages for clocked applications such as quantum cryptographic systems, for applications where one wishes to avoid detection of a strong flash of light that may precede a signal of interest, or for applications where it is desired to obtain accurate time-variation data.
An important consideration when an APD is used for detecting a single photon is that a fraction of the electrons that flow through the APD during an avalanche becomes trapped in defect or impurity states lying within the bandgap of the semiconductor. The trapped electrons can later be excited into the conduction band by thermal fluctuations, thus triggering spurious avalanches. To minimize the amount of trapped charge, it is highly desirable to minimize the total charge passing through the APD due to both photodetection events and dark counts. Pulse biasing inherently suppresses dark counts between the bias pulses by reducing the bias below V.sub.br, and is particularly suited for InGaAs-type APD detectors because these type of detectors exhibit very high dark count rates when operated with constant bias.
A pulsed-bias technique for detecting a single photon at 1.3 .mu.m using a Ge APD is disclosed by, for example, B. F. Levine et al., Pulse biased APD's, Appl. Phys. Lett., 44(5), p. 553, 1984. Relatively long bias pulses (.about.10 ns) were used with a maximum pulse repetition rate of 1 MHz. Subsequently, a short bias pulse (.about.1 ns) and a pulse repetition rate of 10 MHz is disclosed by B. F. Levine et al., Pulse biased APD's, Appl. Phys. Lett., 44(6), p. 581, 1984. Eventually, the pulse repetition rate was increased to 45 MHz, as disclosed by B. F. Levine et al., Pulse biased APD's, Electron. Lett., 20(6), p. 270, 1984. As another example, U.S. Pat. No. 4,754,131 issued Jun. 28, 1988, to Bethea et al. relates to use of APD's for detection of small numbers of photons using APD's.
FIG. 1 shows a voltage waveform diagram for biasing an avalanche photodiode for pulsed-biased single-photon counting. FIG. 2 shows a schematic diagram of a conventional APD detector circuit 20 that provides pulse-biasing for an avalanche photodiode. APD detector circuit 20 includes a coupling capacitor C1 and a resistor R1 that are both connected to the cathode of an avalanche photodiode APD1. The anode of avalanche photodiode APD1 is connected to a signal common through a load resistor R.sub.L. A DC bias voltage V.sub.DC is applied to the cathode of avalanche photodiode APD1 through a resistor R1 so that avalanche photodiode APD1 is reverse-biased below the reverse breakdown voltage V.sub.br of avalanche photodiode APD1. A pulse bias voltage V.sub.pulse is applied through coupling capacitor C1. When a photon 21 is incident on avalanche photodiode APD1, the output signal of APD detector circuit 20 appears across load resistor R.sub.L, which is typically a transmission line having a 50 Ohm characteristic impedance.
A key problem with a pulsed-bias technique is distinguishing a photon-induced avalanche from the large electrical transient caused by the bias pulse voltage. On the one hand, it is desirable to keep the DC bias voltage V.sub.DC well below the reverse breakdown voltage V.sub.br of an APD and use a large amplitude pulse to bring the total bias voltage above V.sub.br. On the other hand, when the bias pulse V.sub.pulse is applied to avalanche photodiode APD1 through the conventional pulse-bias circuit shown in FIG. 2, the capacitance of avalanche photodiode APD1 produces a transient response across load resistor R.sub.L that is essentially the time derivative of bias pulse V.sub.pulse. Avalanche photodiode APD1 is most sensitive at the peak of the bias pulse (corresponding to the zero crossing of the transient), so that when the photon-induced signal is smaller in amplitude than the capacitive transient, it will be "buried" in the transient. FIG. 3 shows a representative waveform diagram of a photon-induced signal 31 that is "buried" in a capacitive transient 32 of a pulse bias signal.
The biasing conditions used by Levine et al. made the photon-induced signals greater than the capacitive transient so that a discriminator could be set to respond only to the photo-induced signals. Specifically, the DC bias voltage V.sub.DC was only .about.0.2 V below the reverse breakdown voltage V.sub.br of the APD and a V.sub.pulse signal .gtoreq.2 V was used for driving the APD well above V.sub.br, resulting in avalanches that moved relatively large amounts of charge (.about.4 pC) through the APD. Transporting such a large amount of charge has the disadvantage as noted above, that the relative amount of trapped charge and the dark count rate increase accordingly.
As mentioned, photomultipliers (PMTs) and multichannel plate (MCP) detectors are used for detecting single photons. The gain of PMT and MCP detectors increase exponentially with an increasing bias voltage and, consequently, can be temporarily switched on and off by applying a bias pulse voltage. Depending on the sign of the bias pulse, the gain of a PMT or a MCP device can be changed, or switched, to higher or lower levels.
PMTs are fabricated to have a cathode that yields electrons in response to incident light, and an electron multiplier formed by of a chain of dynodes. Each dynode yields multiple electrons for each incident electron. An anode at the end of the dynode chain collects electrons to form an output pulse that is typically a few nanoseconds wide resulting from variations in the electron transit times. In general, a single voltage is applied to a PMT that is divided by a resistor chain to provide a stepwise increase in voltage across the dynode chain. The gain of the photomultiplier can be switched by pulsing the overall voltage applied across the dynode chain to provide near-ideal gain switching having on/off gain ratios as high as 10.sup.6 :1.
Applying a voltage pulse across the dynode chain, though, generates an undesirable pulse at the output by capacitive coupling the voltage pulse through the PMT that has such a significant amplitude that the technique of applying a voltage pulse across the dynode chain is rarely used. More commonly, the gain of a PMT is switched by pulsing the voltage applied to the cathode of a PMT, a first dynode of the dynode chain, selected groups of dynodes, or to a focussing electrode. See, for example, D. S. Hanselman et al., Side-on Photomultiplier gating system for Thompson scattering and laser-excited atomic fluorescence spectroscopy, Applied Spectroscopy, Vol. 45, p. 1553, 1991.
The ability to gate a PMT device is critical in applications such as LIDAR and other detection systems that are based on laser-excited fluorescence. In these types of systems, the intensity of the light that is to be measured is often significantly lower than the intensity of the excitation light. Consequently, a PMT is often gated for preventing the PMT from becoming saturated so time-critical measurements of the lower-intensity light level can be made.
Multichannel plates (MCPs) are sensitive to light, but are generally used for detecting particles such as ions or metastable atoms, or for detecting x-rays. MCPs are formed by a "bundle" of numerous micron-sized channels that each act as a separate electron multiplier. The resistance along each channel varies to provide a voltage gradient, so that only a single voltage is applied across a single plate. Single plates or pairs of plates are used so that the output of one plate is the input to a second plate, thus providing higher overall gain. In the case of a two-plate detector, the gain is switched by switching the voltage applied across the first plate.
MCPs are also gated for improving detection efficiency. A major concern is to keep the gain low during an initial laser (or other beam) pulse. Transit-time broadening is much less, relatively speaking, for an MCP because an MCP is much shorter in length than a PMT. For example, MCP pulses can be as short as 100 ps, consequently, MCPs can be gated on a timescale of this magnitude. Such gating is commonly used to allow very fast events, such as laser-induced detonations, to be followed in time.
What is needed is a way for detecting small charge pulses when using a pulsed-biasing technique for an avalanche photodiode, a photomultipler or a multichannel plate detector circuit.